Signal transition enhancement circuitry for use in color television signal processing apparatus is known from, for example, U.S. Pat. No. 4,777,385 and the article by Hartmut Harlos, "Picture Signal Improvement In Colour TV Receivers", published in the IEEE Transitions on Consumer Electronics, Vol. CE-31, No. 3, August 1985, both of which are incorporated herein by reference. The Harlos article describes an analog processing approach to transition enhancement and U.S. Pat. No. 4,777,385 describes primarily a digital approach to transition enhancement. In both systems, signal transitions having slopes greater than a predetermined value are detected. Responsive to the detected transitions, circuitry is employed to hold the initial amplitude value of the transition for a portion of the duration of the transition and thereafter the final value of the signal transition is coupled to the signal output device. The transition is shortened to the time required to switch between the initial and final values of the transition, which time is typically much shorter than the original transition time. The Harlos system accomplishes this function by incorporating a series switch in the signal path followed by a storage capacitor having one terminal coupled to a point of fixed potential. The series switch is maintained closed until a transition is detected, at which time the switch is opened. The value of the signal at the beginning of the transition is stored on the capacitor and output to further processing circuitry. A predetermined time after the switch is opened it is returned to the closed condition to couple the incoming signal to the capacitor and the further processing circuitry.
In U.S. Pat. No. 4,777,385, input signal, represented by, for example, binary samples, is applied to a serial-input-parallel-output shift register. The parallel outputs are coupled to respective inputs of an N-input-to-one-output multiplexer. A modulo N counter is coupled to address the multiplexer. In the absence of transitions the multiplexer is conditioned to pass signal from the center tap of the shift register. Responsive to the detection of a signal transition, the counter is enabled to condition the multiplexer to successively pass signal samples from the center tap and subsequent taps toward the end of the shift register, then to successively pass samples from a tap at the beginning of the shift register and subsequent taps toward the center tap. In this manner transients are compressed to a single sample period.
The drawback of the foregoing transition enhancement systems is that all detected signal transitions are compressed to the same transition time. This feature tends to undesirably distort some signal components rather than enhance them. The present invention is arranged to variably enhance signal transition according to a feature of the particular transition detected.